This invention relates to a sintered member formed of an aluminum nitride base composite material, which contains whiskers of silicon carbide as a reinforcing and hardness enhancing ingredient. The sintered member may be a cutting tool.
Aluminum nitride, AlN, possesses good heat conductivity and is highly resistant to heat and thermal shocks and also to corrosion by molten metals. Accordingly, in the field of new ceramics AlN has been attracting increasing attention, like Si.sub.3 N.sub.4 and SiC, as a principal material of various sintered parts including structural members to be exposed to high temperatures.
In practice, sintering of pure AlN powder encounters problems and difficulties. Therefore, some kinds of sintering assistants have been tested for industrial preparation of AlN ceramics. For example, Japanese patent application primary publication No. 50-151704 (1975) shows a method of mass-producing AlN base sintered members by a hot pressing process using adequate amounts of selected sintering assistants, which are represented by Mo and W, Mo.sub.2 C and/or WC, so as not to impair the advantageous properties of AlN. The AlN base sintered members obtained by this method are comparable to conventional alumina base ceramics members in mechanical strength and toughness, but the AlN base ceramics members are lower in hardness than Al.sub.2 O.sub.3 base ceramics members which are often embodied in cutting tools. The hardness of the AlN base sintered members is about 85 at the maximum, in terms of Rockwell superficial hardness 45N scale. (In the present application hardness values are always in this sense unless otherwise noted.)
To compensate for insufficient hardness of AlN base ceramics, Japanese patent application primary publication No. 54-31411 (1979) proposes an AlN base sintered member which has a hard alumina coating in at least a selected area of the surfaces and can serve as a cutting tool. However, the provision of such alumina coating inevitably leads to lowering of productivity and a considerable increase in the production cost.